Reerror: error-resilient parsing (same as Merlin for OCaml)

PLAN

@@ -0,0 +1,27 @@
+Fix error recovery & error reporting
+
+Step 1: remove existing implementation
+
+OK  * Put reason parser in its own file
+OK  * Remove menhir error management:
+OK    - remove "error" token 
+OK    - remove error messages infrastructure
+OK    Now message defaults to "Syntax error"
+    * Fix reported error location (when recovering or not)
+
+Step 2: reintroduce recovery
+    * Preprocess grammar:
+      - check exhaustivity of recovery
+      - produce a mapping of automaton states to automaton-items suitable for
+        recovery
+    * Instrument parser:
+      - first, always complete the AST and drop user input (":'(")
+      - second, introduce an heuristic for recovering based on location
+
+Step 3: reintroduce error messages
+    * Ask the crowd: 
+        What should messages look like? 
+        Which situations are tricky or counter-intuitive?
+    * Make a testsuite representative of common syntax errors 
+    * ... Design an analysis sufficient to produce the messages automatically
+          :P

esy.json

@@ -5,6 +5,7 @@
   "version": "3.5.0",
   "dependencies": {
     "ocaml": " >= 4.2.0 < 4.9.0",
+    "@opam/fix": "*",
     "@opam/ocamlfind": "*",
     "@opam/menhir": " >= 20170418.0.0",
     "@opam/utop": " >= 1.17.0 < 2.3.0",

formatTest/errorTests/expected_output/reservedField.re

@@ -1,2 +1,2 @@
 File "reservedField.re", line 1, characters 11-15:
-Error: 1054: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedField.re.4.08.0

@@ -1,5 +1,5 @@
 File "reservedField.re", line 1, characters 11-15:
 1 | let x = {< type >};
                ^^^^
-Error: 1054: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
 

formatTest/errorTests/expected_output/reservedRecord.re

@@ -1,2 +1,2 @@
 File "reservedRecord.re", line 1, characters 24-28:
-Error: 1863: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecord.re.4.08.0

@@ -1,5 +1,5 @@
 File "reservedRecord.re", line 1, characters 24-28:
 1 | let foo = { foo: "bar", type: "qux" };
                             ^^^^
-Error: 1863: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
 

formatTest/errorTests/expected_output/reservedRecordPunned.re

@@ -1,2 +1,2 @@
 File "reservedRecordPunned.re", line 1, characters 22-26:
-Error: 3411: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordPunned.re.4.08.0

@@ -1,5 +1,5 @@
 File "reservedRecordPunned.re", line 1, characters 22-26:
 1 | let foo = { ...other, type };
                           ^^^^
-Error: 3411: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
 

formatTest/errorTests/expected_output/reservedRecordTy

@@ -1,2 +1,2 @@
 File "reservedRecordTypePunned.re", line 1, characters 11-15:
-Error: 770: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordType.re

@@ -1,2 +1,2 @@
 File "reservedRecordType.re", line 1, characters 11-15:
-Error: 770: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordType.re.4.08.0

@@ -1,5 +1,5 @@
 File "reservedRecordType.re", line 1, characters 11-15:
 1 | type x = { type: string };
                ^^^^
-Error: 770: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
 

formatTest/errorTests/expected_output/reservedRecordTypePunned.re

@@ -1,2 +1,2 @@
 File "reservedRecordTypePunned.re", line 1, characters 11-15:
-Error: 770: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordTypePunned.re.4.08.0

@@ -1,5 +1,5 @@
 File "reservedRecordTypePunned.re", line 1, characters 11-15:
 1 | type x = { type };
                ^^^^
-Error: 770: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
 

reason.opam

@@ -17,6 +17,7 @@ depends: [
   "ocamlfind"     {build}
   "menhir"        {>= "20170418"}
   "merlin-extend" {>= "0.4"}
+  "fix"
   "result"
   "ocaml-migrate-parsetree"
 ]

src/menhir-recover/Makefile

@@ -0,0 +1,15 @@
+.PHONY: all clean
+
+all:
+	dune build main.exe
+
+clean:
+	dune clean
+
+test-dijkstra:
+	dune build main.exe demo/calc.exe
+	_build/default/main.exe _build/default/demo/parser.cmly
+
+test-custom:
+	dune build main.exe demo/calc.exe
+	_build/default/main.exe -custom _build/default/demo/parser.cmly